1. Technical Field
The present invention relates to solar power plants. More particularly, the invention relates to a high concentration central receiver system having improved reflectors and a unique heat removal system.
2. Discussion
As concerns over the environment, the deterioration of fuel sources, and energy efficiency continue to increase, solar power plants have become the subject of worldwide attention. In the development of solar power plants, high concentration central receiver systems have demonstrated a relatively high level of usefulness and are therefore quite popular. The conventional solar central receiver system has a xe2x80x9ctower topxe2x80x9d configuration in which a field of heliostats reflect sunlight onto a receiver mounted on a tower structure. The concentrated solar energy on the receiver heats a fluid, such as oil or molten salt, to high temperatures. This energy is then transferred to a boiler/heat exchanger to produce steam, which then powers a steam turbine to produce electricity. While this type of configuration has been shown to be useful for power plants, other configurations have proven to be more effective for large power plants, especially when operated with high efficiency, combined cycle gas turbines powered by both natural gas and solar energy.
One such configuration is the xe2x80x9ctower reflectorxe2x80x9d configuration. One of the major features of this type of configuration is that special parabolic concentrators are located on the ground beneath the tower and reflectors are coupled to the tower structure at a predetermined height above ground for reflecting solar radiation. The tower-mounted reflectors redirect sunlight from the heliostats, to the parabolic concentrators which are located on the ground. The tower-mounted reflector is composed of a number of mirrors, coupled to a metallic facet (or heat exchanger) for support. Each parabolic concentrator typically has a special quartz receiver into which the concentrated light is directed. Air flowing through this receiver is air heated to a high temperature and then passes into the turbine combustion chamber, where it is further heated, before passing through the turbine to produce electric power by turning the generator.
It is critical that the tower mounted reflectors provide the light into the aperture opening of the parabolic concentrators at the appropriate angles under a wide variety of conditions. These conditions include temperature changes, wind variations, solar insolation levels, sun angles, etc. It is very desirable that the concentrators have very little loss due to xe2x80x9cspillagexe2x80x9d under these conditions, because conventional systems make no use of this wasted heat. The tower reflectors must therefore achieve high optical quality at a low cost. The tower reflectors must also be able to withstand high concentrations of solar energy and meet the optical requirements under a wide variety of environmental conditions. It is therefore desirable to provide reflectors having a good structural integrity and that are safe to operate. It is also desirable to enable the reflectors to be adjustable and configurable such that there is minimal loss of reflected light from the heliostats in harsh environments and over several decades.
A particularly difficult aspect of conventional solar reflectors relates to high operating temperatures, cost and breakage. Specifically, while various facet designs and heat removal systems have been designed for tower reflectors, a number of difficulties remain. For example, the conventional design has a small reflector area and uses small, high tensile strength, thick glass mirrors. Generally, these mirrors have been shown to be too costly for practical use in high temperature commercial applications. The conventional design is also prone to breakage, since the glass is held by xe2x80x9cclipsxe2x80x9d such that there are slight stresses built up in the glass under nominal conditions. It is therefore easy to understand that such systems can impose relatively high levels of stress at local points under more severe conditions. For example, high stresses occur (especially when exposed to sand, dust and ice, since these can cause xe2x80x9cratchetingxe2x80x9d) when the glass expands and contracts due to exposure to diurnal cycles of high concentration irradiance, with high temperatures, followed by little or no irradiance and relatively cool temperatures.
The resultant expansion and contraction, with metal joints used to hold the glass securely for good alignment, can result in high local stresses and breakage. Since the glass is not otherwise constrained, it can fall, causing a significant hazard to equipment and personnel below. In particular, the falling glass can damage the high optical quality, relatively high cost Compound Parabolic Concentrators (CPCs) on the ground. These thermal and stress related problems are exacerbated further by the exposed clips, which can be subjected to over 50 to 100 suns (i.e., 50 to 100 kW/m2). Since the metal has a relatively high solar absorptivity, the operating temperature of the metal clips can be quite high, thus adding to the local thermal stresses already placed on the facets by the direct, concentrated solar flux.
The conventional approach also does not provide for adequate thermal control to prevent ice buildup. Ice buildup on high structures is a serious problem, since it can greatly increase the structural load, distort and damage the glass mirrors. If ice forms and falls onto the CPCs, further damage is likely to be caused to the system. It is therefore desirable to provide a design that ensures thermal control to prevent buildup.
Another aspect of the conventional design is that it uses a rectilinear support structure. Such supports do not offer the torsional stiffness inherent in geometries such as triangular shapes. The mass of material required, and the complexity of assembly (as well as cost) are therefore higher than for other geometric shapes. For example, the triangular design disclosed herein, is formed with a novel xe2x80x9cgeodesic domexe2x80x9d concept, that uses essentially equilateral struts arranged with novel attachment fittings to allow easy assembly of the support structure and adjustment of the facets.
In the more general case, for certain applications, mirrors are heated by incident solar irradiance and/or heat flux. This heating can cause damage to the mirrors or to the support backing structure. Furthermore, the optical quality can be degraded by changes in the radius of curvature, increases in the surface slope error, damage to the reflective surface, or warpage. The problem is typically solved by either selecting high tensile strength glass (at high cost), flowing a stream of air over the mirror, or using a fluid coolant. It is important to note that while conventional coolant-based heat removal systems are moderately effective in sinking heat away from the reflectors, other shortcomings remain. For example, the xe2x80x9cspillagexe2x80x9d area immediately adjacent the concentrators is also a considerable source of heat. Removing heat from this area would both improve the operation of the concentrators as well as provide additional heat to other systems (e.g., residential/commercial systems). As already mentioned, extreme cold or ice buildup can also cause problems. These problems include warpage of the facet or its support structure, changes in the facet cant angle, build-up of extremely high loads on the structure, or cracks in the glass. To mitigate concerns of extreme cold and ice buildup, an embodiment of the invention utilizing a fluid coolant heat recovery system can maintain adequate coolant temperatures to prevent formation of ice and protect the area from extreme cold.
In general, the mirrors must be adjusted to produce the beam positioning required by the application. This problem is typically solved by attaching multiple (most often, three) adjustable attachment fittings to the back of the mirror assembly. Also, for certain applications, the mirror assembly must be very light weight, and in some applications the mirror must be mounted in a location where access is difficult. For example, in the tower reflector case, the reflectors are mounted high above the ground (hundreds of feet high). Therefore, the reflectors must demonstrate exceptional long life and integrity, while at the same time being light weight and inexpensive.
The above and other objectives are provided by a method and high concentration central receiver system in accordance with the present invention. The central receiver system has a plurality of interconnected reflectors coupled to a tower structure at a predetermined height above ground for reflecting solar radiation. A plurality of parabolic concentrators are disposed between the reflectors and the ground such that the concentrators receive reflected solar radiation from the reflectors. The central receiver system further includes a heat removal system for removing heat from the tower-mounted reflectors and an area immediately adjacent the parabolic concentrators. Removing the heat from the area immediately adjacent the concentrators improves operation of the concentrators and provides an additional source of energy that is effectively wasted in conventional systems.
Further in accordance with the present invention, a reflector for a high concentration central receiver system is provided. The reflector includes a mirror, a facet, and an adhesive compound. The facet has walls defining a coolant channel, where the cooling channel receives a heat conductive fluid. The adhesive compound is disposed between the mirror and the facet such that the mirror is fixed to the facet under compressive stresses. In the preferred embodiment, the glass mirror has a compression stress value such that no part of the glass experiences tensile stresses. Generating compressive stresses in the mirror improves the strength and resistance to breakage because glass has a low tensile strength. Pre-loading the glass in compression thus avoids the most common failure mode for glass.
In another aspect of the invention, a method for fabricating a reflector for a high concentration central receiver system is provided. The method includes the step of maintaining a mirror at a mirror bonding temperature. A metal facet is maintained at a facet bonding temperature, where the facet bonding temperature is greater than the mirror bonding temperature. The method further includes the step of bonding the mirror to the facet with an adhesive, where an operating temperature for the reflector is less than the facet bonding temperature. Due to the inherent properties of glass and metal, the mirror is under compressive stresses at the operating temperature.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.